This measure divides the age distribution into two equal parts: one-half of the cases falling below the median value and one-half above the value. Median age is computed on the basis of a single year of age distribution.

The most general limitation for many decades has been the tendency of people to overreport ages or years of birth that end in zero or five. This phenomenon is called "age heaping." In addition, the counts in the 1970 and 1980 censuses for people 100 years old and over were substantially overstated. So also were the counts of people aged 69 in 1970 and aged 79 in 1980. Improvements have been made since then in the questionnaire design, and in the allocation procedures which have further minimized these problems. The count of people aged 89 in the 1990 census was not overstated.

Review of detailed 1990 census information indicated that respondents tended to provide their age as of the date they completed the questionnaire, not their age as of April 1, 1990. One reason this happened was that respondents were not specifically instructed to provide their age as of April 1, 1990. Another reason was that data collection efforts continued well past the census date. In addition, there may have been a tendency for respondents to round their age up if they were close to having a birthday. It is likely that approximately 10 percent of people in most age groups were actually one year younger. For most single years of age, the misstatements were largely offsetting. The problem is most pronounced at age zero because people lost to age one probably were not fully offset by the inclusion of babies born after April 1, 1990. Also, there may have been more rounding up to age one to avoid reporting age as zero years. (Age in complete months was not collected for infants under age one.)

The reporting of age one year older than true age on April 1, 1990, is likely to have been greater in areas where the census data were collected later in calendar year 1990. The magnitude of this problem was much less in the 1960, 1970, and 1980 censuses where age was typically derived from respondent data on year of birth and quarter of birth.

These shortcomings were minimized in Census 2000 because age was usually calculated from exact date of birth and because respondents were specifically asked to provide their age as of April 1, 2000. (For more information on the design of the age question, see the section below that discusses "Comparability.")

Age data have been collected in every census. For the first time since 1950, the 1990 data were not available by quarter year of age. This change was made so that coded information could be obtained for both age and year of birth. In 2000, each individual has both an age and an exact date of birth. In each census since 1940, the age of a person was assigned when it was not reported. In censuses before 1940, with the exception of 1880, people of unknown age were shown as a separate category. Since 1960, assignment of unknown age has been performed by a general procedure described as "imputation." The specific procedures for imputing age have been different in each census. (For more information on imputation, see "Accuracy of the Data.")
For more information on age, please telephone 301-457-2428.